This invention relates to steam turbines and more particularly to means for diminishing exhaust pipe erosion in steam turbines. Moisture leaving the exhaust system of a steam turbine generator, typical of a high pressure steam turbine used in nuclear plants, may cause erosion of the cross-under piping which connects the turbine exhaust hood and the moisture separator reheater. Exhaust pipes which connect the high pressure turbine with the moisture separator reheater in a nuclear power plant are subject to serious erosion damage. Damage is caused by the high velocity impact of coarse water droplets which have diameters of the order of 100 .mu.m or greater. In pressurized water reactor steam generating systems saturated steam is produced at about 1,000 psig. Moisture is formed immediately as the expansion process begins in the high pressure stage of the turbine. The high pressure stage exhaust typically contains 11% by mass water. Before entering the low pressure stage the wet steam passes from the high pressure turbine exhaust hood to a moisture separator reheater via exhaust piping. Exhaust pipes leaving the exhaust hood at a low elevation are known as cross-under pipes. All current models of nuclear plant high pressure turbines have at least two cross-under pipe exhausts. Some models have a third exhaust which leaves the top center of the exhaust hood. This is also termed a crossunder pipe.
Erosion of cross-under exhaust pipes is a common problem in nuclear plants. It is believed that the erosion of the exhaust pipes is caused by the high velocity impact of moisture droplets. Erosion studies indicate that erosion damage is a very strong function of the droplet diameter in the range of diameter of 1 .mu.m through 1,000 .mu.m, and it is likely that all of the damage is caused by droplets in the size range of above 10 .mu.m. It has been found that within a typical exhaust hood the wet steam turns abruptly within the hood and the hood acts as a very effective moisture separator. Present exhaust hoods, though, are not equipped with a means of collecting the water. Generally, in present exhaust hoods the water film which forms on the hood walls is swept toward the exhaust nozzles, whereupon the water film becomes re-entrained by the steam flow in the form of relatively large droplets. These droplets might be of order of diameter 1,000 .mu.m.
In a typical high pressure nuclear turbine, i.e. having an inlet pressure of 1000 psig, as the flow leaves the last blade row of the high pressure turbine a fraction of the mass which flows as liquid is about 11%. Since the total flow entering a hood is typically 6 million lb/hr 11% moisture constitutes a moisture flow rate of 6.6.times.10.sup.5 lb/hr. The fraction of the liquid which exists as small droplets (order of 1 .mu.m diameter or less) and that which is in the form of large droplets (order of 100 .mu.m diameter or greater) is problematical. Estimates place the large droplet fraction between 10 and 30%. This of course depends upon the location of the last liquid extraction point in the turbine. Even if the smallest estimate is used, about 66,000 pounds per hour of water in large droplet form enter each exhaust hood, however; measurements made at a nuclear power station suggest that 198,000 lb/hr (30% of total water) is a more representative figure.
As the flow leaves the annular expansion passage (blade ring) it divides and turns within the exhaust hood so that it might leave the hood through one of the exhaust nozzles. During this process centrifugal action tends to separate the moisture from the steam and deposit it on the walls. It has been calculated that the relatively small or primary droplets which are assumed to have a mean diameter of 1 .mu.m, of these droplets 0.12% are centrifuged onto the walls. In a typical system it is estimated that the primary droplet flow rate is on the order of 4.62.times.10.sup.5 lb/hr in each hood (70% of total moisture flow). The 0.12% of primary moisture separated accounts for 554 lb/hr of water deposited on the walls. In contrast to this, if coarse water droplets are assumed to have a mean diameter of 100 .mu.m, 68% of these coarse droplets are centrifuged onto the walls of the exhaust hood. This accounts for 1.35.times.10.sup.5 lb/hr of water deposited on the walls. The total amount of water centrifuged out of the flow within each hood may be taken to be 1.35.times.10.sup.5 lb/hr.
Water deposited on the exhaust hood walls will be swept toward the exhaust nozzles. The principal mechanism for movement of the water is the sheering action imparted by the steam flow. Gravitational action is of secondary importance. Consequently, almost as much water is swept towards the vertically upward facing nozzle in a system that has such a nozzle as is swept toward each of the nozzles which point at a downward angle. Unless means are provided to collect the water on the exhaust walls, it will be sheered off the rim of the exhaust nozzles and will become re-entrained in the flow as it enters the exhaust pipes, creating a high mass flux of hypercoarse water. This hypercoarse water has a diameter range of several hundred through a thousand microns, which is quickly accelerated by the exhaust pipe steam to a velocity on the order of 250 ft/sec. It has been found that the rate of erosion on the exhaust pipes is a strong function of droplet diameter and droplet velocity.
A further compounding problem occurring in the typical exhaust piping between the turbine and the moisture separator reheater occurs at the pipe bends. In a typical exhaust pipe system between the turbine and the moisture separator reheater, the pipe bends are provided with turning vanes which are included to decrease pressure loss at the bends. Even if means are provided for collecting the liquid forming off the exhaust hood walls about 70% of the liquid will pass through the nozzles as small primary droplets and about 9.6% of the liquid will pass through the nozzles as large secondary droplets. Some of the benign primary droplets will be transformed into potentially damaging large droplets as the flow negotiates bends and turning vanes. It has been estimated that 11% of the flow in a high pressure exhaust hood (6.6.times.10.sup.6 lb/hr), is moisture and that typically 30% of this moisture flow is in the form of large droplets. If means are provided to remove the 68% portion of this flow which is deposited on the exhaust hood walls the moisture flow leaving the exhaust hood will be composed of 6.3.times.10.sup.4 lb/hr secondary droplets and 4.62.times.10.sup.5 lb/hr primary droplets. If the exhaust hood feeds equally into three exhaust pipes the moisture flow rates per pipe will be 2.1.times.10.sup.4 lb/hr secondary droplets and 1.54.times.10.sup.5 lb/hr primary droplets. Calculations indicate that 4600 lb/hr of the primary droplet flow will be centrifuged onto the turning vanes at each elbow and will be sheared off the trailing edges of the vanes in the form of damaging secondary droplets. It is further estimated that these droplets will impact the pipe within a distance of 5 feet downstream of the bend.
An object of the present invention is to provide means for collecting the damaging large droplets and prevent them from becoming re-entrained in the steam flow and thus reduce the incidence of pipe erosion.